The invention relates to a device for dewatering a pulp web, with a dewatering section comprising a top and a bottom wire where there is a first wedge-shaped dewatering zone which can be adjusted and/or pressure-loaded at the end, and with a pair of press rolls mounted at the end of the bottom wire.
A device of this kind is known from AT 405538B, for example. Here a device is already described with a wedge-shaped dewatering zone and which is adjustable at the end. In this case a pulp suspension is fed in with a consistency of, for example, approximately 1-1.5% and the pulp web has a dry content in the range of approximately 12-14% DS (dry solids) at the end of the wedge. Further measures, particularly the use of a heavy duty press after the dewatering press, are applied in attempts to obtain the highest possible dry content in the pulp web. This is particularly important for the subsequent thermal drying stage, which requires less energy input with higher dry content levels at the inlet. The demands made on the dewatering units rise constantly in terms of throughput and dewatering performance, not least in order to minimize the specific costs.
The aim of the invention is to create a device that can be used for all different kinds of products and achieve the highest possible dry content levels.
The invention is thus characterized by a shoe press roll being used as the top roll of the press roll pair at the end of the bottom wire. By using a shoe press roll it is possible to apply higher pressing forces at this point and for a longer period, which results in a corresponding increase in dry content.
An advantageous further development of the invention is characterized by a felt running over the shoe press roll. With this arrangement even more water can be removed from the press zone.
A favorable further development of the invention is characterized by the bottom roll having an outer shell with holes and inner roll shell with grooves in longitudinal direction. With this configuration the water drains particularly well out of the press zone, thus permitting a substantially better dewatering effect by pressing.
An advantageous configuration of the invention is characterized by suction boxes being mounted on the face ends of the bottom press roll. This permits even faster and better drainage of liquid from the press zone.
A favorable configuration of the invention is characterized by the bottom press roll in the press roll pair at the end of the bottom wire being built with deflection compensation elements along the length of the roll. As a result it is possible to apply higher line pressures (up to 1500 N/m working width) with the shoe press roll, particularly with larger working widths, thus achieving a higher dry content than with a conventional roll press.
A favorable further development of the invention is characterized by at least one further pair of press rolls being provided after the wedge-shaped dewatering zone. A higher dry content, which is beneficial to further dewatering, can be achieved by placing one, two or, for example, three additional pairs of press rolls after the wedge zone.
A favorable configuration of the invention is characterized by the press rolls being designed with a smooth surface, however they may also have a grooved surface as an alternative.
A favorable further development of the invention is characterized by a web heating device, for example a steam blow box, being provided ahead of the shoe press roll. By including a web heating device the viscosity of the water contained in the web is reduced, thus facilitating dewatering by pressing in the subsequent shoe press, which leads in turn to higher dry content levels.
An advantageous configuration of the invention is characterized by supported web transfer after the shoe press roll by a suction roll to a second shoe press, where one felt can be run round the shoe press roll and one round the deflection compensation roll in the second shoe press.
A favorable configuration of the invention is characterized by the entire device being suitable for cantilevering. Since the machine can be fully cantilevered, wires and felts can be changed easily.